Commit 28423ad2 authored Jan 13, 2015 by Calvin Owens Committed by Paul E. McKenney Jan 14, 2015

ksoftirqd: Enable IRQs and call cond_resched() before poking RCU



While debugging an issue with excessive softirq usage, I encountered the
following note in commit 3e339b5d ("softirq: Use hotplug thread
infrastructure"):

    [ paulmck: Call rcu_note_context_switch() with interrupts enabled. ]

...but despite this note, the patch still calls RCU with IRQs disabled.

This seemingly innocuous change caused a significant regression in softirq
CPU usage on the sending side of a large TCP transfer (~1 GB/s): when
introducing 0.01% packet loss, the softirq usage would jump to around 25%,
spiking as high as 50%. Before the change, the usage would never exceed 5%.

Moving the call to rcu_note_context_switch() after the cond_sched() call,
as it was originally before the hotplug patch, completely eliminated this
problem.

Signed-off-by: Calvin Owens <calvinowens@fb.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

parent 5a43b88e

kernel/softirq.c

+5 −1

Original line number	Diff line number	Diff line
		@@ -656,9 +656,13 @@ static void run_ksoftirqd(unsigned int cpu)
		* in the task stack here.
		*/
		__do_softirq();
		rcu_note_context_switch();
		local_irq_enable();
		cond_resched();

		preempt_disable();
		rcu_note_context_switch();
		preempt_enable();

		return;
		}
		local_irq_enable();

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